The jail itself has no overhead, in terms of CPU, RAM, etc but these are directly dependent on the application running inside the jail. For instance, if you plan on having a large NAS, expect high HDD utilization. On the other hand, if you plan on running munin inside the jail (with many nodes), expect high CPU usage (due to graph computation).
Again, the amount of resources used by the jail is directly dependent on the application(s) running inside it.